tools/power/turbostat: Remove Package C6 Retention on Ice Lake Server

Currently the turbostat treats ICX the same way as SKX and shares the
code among them. But one difference is that ICX does not support Package
C6 Retention, unlike SKX and CLX.

So this patch:

1. Splitting SKX and ICX in turbostat.
2. Removing Package C6 Rentention for ICX.

And after this split, it would be easier to cutomize Ice Lake Server
in turbostat in the future.

Suggested-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: Chen Yu <yu.c.chen@intel.com>
Reviewed-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Tested-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>